(a) Technical Field
The present disclosure relates to a method of forming a nanostructured surface (NSS) on a polymer electrolyte membrane (PEM). IN preferred aspects, it relates to a method of forming a nanostructured surface (NSS) on a polymer electrolyte membrane (PEM) of a membrane electrode assembly (MEA) for a fuel cell, in which a nanostructured surface such as nano-hair structures is formed on a polymer electrolyte membrane by plasma treatment with gases, for example, argon, oxygen, CF4, by plasma-assisted chemical vapor deposition (PACVD), where catalyst particles or a catalyst layer is directly deposited on the surface of the polymer electrolyte membrane having the nanostructures, for example Inano-hairs or nano-holes, by sputtering, thus suitably fabricating a membrane electrode assembly for a fuel cell by a simple process and considerably reducing the amount of catalyst needed in the method.
(b) Background Art
Polymer electrolyte membrane fuel cells (PEMFC) that are presently used have many advantages, including low operation temperature and high energy efficiency, when compared to other types of fuel cells. Accordingly, extensive research aimed at utilizing the PEMFC as a power source for vehicles has been carried out.
In the PEMFC, the current density is high, the operation temperature may be as low as 60 to 80° C., and the possibility of corrosion and the electrolyte loss are considerably low, for example when compared to a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), or a solid oxide fuel cell (SOFC).
Further, because a PEMFC can be produced at low cost, with a low volume, has a long stack life, and fast start-up, is suitable for discontinuous operation, and is able to stably supply power, a PEMFC has various applications, such as in vehicles.
Each of the unit cells constituting the fuel cell preferably includes a polymer electrolyte membrane (or a proton exchange membrane), and an oxidation electrode (anode) and a reduction electrode (cathode) that are suitably integrally formed on both sides of the polymer electrolyte membrane by a hot press, thus forming a membrane electrode assembly (MEA) 10.
Preferably, since the anode and the cathode are integrally formed on the surface of the polymer electrolyte membrane, a reaction [H2→2H+2e+] takes place at the anode, on which carbon black as a catalyst support, and where water and protons can be bonded, is suitably coated.
At the cathode, a reaction [½O2+2H+2e+→H20] takes place, and protons are transported through the MEA by the above reaction, thus generating electricity and water.
Preferably, a polyperfluorosulfonate polymer electrolyte membrane having high hydrogen ion conductivity is used for the polymer electrolyte membrane of the fuel cell, and, mor preferably, a Nafion membrane manufactured by Dupont is more widely used, since the surface has hydrophobic properties and is structurally stable.
As research related to the polymer electrolyte membranes for fuel cells has progressed, the stability of the fuel cell and its manufacturing cost have become objects of research, as the properties of the polymer electrolyte membrane and the amount of used catalyst are closely related to the performance of the fuel cell and its manufacturing cost.
Accordingly, as one of the conventional methods for improving the performance of the MEA and reducing the amount of catalyst loadings, a method of forming a whisker structure on a silicon substrate and attaching it to the surface of the polymer electrolyte membrane using an adhesive has been proposed. Using this method, it is possible to considerably reduce the amount of catalyst loadings due to an increased surface area when platinum catalyst is deposited on the surface of the electrolyte membrane, as compared to the conventional methods.
Further, attempts to improve the performance of the polymer electrolyte membrane by directly modifying the surface of the polymer electrolyte membrane using various methods such as plasma, ion beam, or electron beam, have been made, and efforts to increase the contact area between the polymer electrolyte membrane and catalyst have continued to progress. Moreover, it has been reported that, if the surface area is increased, the amount of expensive catalyst can be reduced and, further, the efficiency can be considerably improved [M. Prassanna, E. A. Cho. H. J. Kim, T. H. Kim, et al., J. Power Sources (2006): Korean Patent No. 0405671 (2002) to Ki Chun Lee at al., both incorporated by reference in their entireties herein].
A method of increasing surface roughness using ion beams has been recently employed, and a method of coating platinum catalyst-supported carbon black on the thus surface-modified polymer electrolyte membrane by spray coating has also been used. In a study aimed at utilizing low pressure and high density argon (Ar) plasma [D. Ramdutt et al, Journal of Power Sources 165 (2007) 41-48, incorporated by reference in its entirety herein], a polymer electrolyte membrane was exposed to argon plasma at a power of 50 W for 5 to 120 seconds; however, the surface had hydrophilic properties and the shape of the surface was little changed, which resulted in a deterioration of proton conductivity.
Accordingly, a study on surface treatment conditions, which provides a method to increase surface area due to a change in surface shape of the membrane electrode assembly and, at the same time, to increase in hydrophobic nature of the surface, is needed.
Research aimed at supporting platinum (Pt) as a catalyst for delivering protons on the surface of a polymer electrolyte membrane, i.e., an ion exchange membrane, has been carried out. Further, in order to reduce the amount of platinum loading, a method of supporting platinum particles on carbon black and fabricating an ion exchange membrane by spray coating, screen printing, or tape casting has been proposed.
Dual ion-beam assisted deposition and sputter deposition, in which the surface of a polymer electrolyte membrane is suitably modified and then platinum is directly deposited on the surface of the membrane by sputtering, and electrodeposition, electrospray, etc. have been proposed [J. H. Wee, K. Y. Lee. S. H. Kim, Journal of Power Sources 165 (2007) 667-677, incorporated by reference in its entirety herein].
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.